1. Field of the Invention
The present invention relates to a dielectric ceramic composition having dielectric properties suitable for microwave regions, such as a low dielectric loss, a large dielectric constant and temperature stability, and capable of being sintered at low temperature. More specifically, the present invention pertains to a dielectric ceramic composition suitable for use in electronic parts such as a resonator, a capacitor, a filter and the like, or electronic parts having inner conductors such as embedded substrates, by sintering with metal electrodes such as silver, copper, and silver/palladium, and a method for preparing thereof.
2. Description of the Prior Art
With great advances in electronic and communication technologies, apparatuses and equipments for embodying them have recently been miniaturized. To this miniaturization, stacking and chipping techniques of electronic parts make a great contribution. Recently, according to the development of communication means using microwave bands, such as mobile phones and satellite broadcasting, particular pressure has been placed on electronic parts made of dielectrics to miniaturize.
Representative of the electronic parts to which stacking techniques are applied is a capacitor. Examples of stacked type electronic parts for use in mobile communication terminals include filters, couplers, duplexers, oscillators and multichip modules (MCM). , The stacked type electronic parts, the most portions of which are composed of multilayer dielectrics and inner electrodes, are fabricated by laminating a dielectric into a tape, printing an inner electrode onto the dielectric laminate, stacking a plurality of the laminates and firing the stack.
To be useful for stacked type elements, accordingly, IS dielectrics must be capable of being sintered along with electrodes in addition to having dielectric properties suitable for application. Such dielectric requirements include high dielectric constant (.di-elect cons..sub.r), quality factor (Q.times.f), and low dependency of resonance frequency modulation on temperature change and the like.
Materials suitable for the inner electrodes are silver, copper, nickel, palladium, platinum, gold and alloys thereof. Selection of the inner electrode materials is made depending on the sintering temperature and properties of the ceramic dielectric used and vice versa.
For example, silver (Ag), showing the lowest specific resistance (1.62.times.10.sup.-4 .OMEGA.cm) and being inexpensive, cannot be applied to ceramic dielectrics which must be sintered at 950.degree. C. or higher because of its low melting point (961.degree. C.). In spite of their high melting points, gold (Au), platinum and palladium (Pd) are restricted in their use because of their being expensive. As for copper (Cu) or nickel (Ni) electrodes, their very poor oxidation resistance requires sintering at an oxygen partial pressure as low as about 10.sup.-9 atm, causing the problem that, when thermally treated under such a low oxygen partial pressure, most dielectric ceramic compositions show highly increased dielectric loss and thus cannot be used as capacitors.
Ceramic dielectric compositions currently used in stacked type electronic parts are mostly based on BaTiO.sub.3, optionally added with oxide sintering aids or glass frits for reduction of sintering temperatures. Typically, these dielectric compositions range, in sintering temperature, from 1,100 to 1,300.degree. C., as well as being resistant to reduction and having dielectric constants of several hundreds or higher. However, their great dielectric loss makes it difficult to apply them where a frequency band of MHz or higher is used. Additionally, the dielectric compositions suffer from the drawback of undergoing a dielectric constant fluctuation of as large as hundreds ppm/.degree. C., which prevents them from being applied to temperature-stable capacitors or electronic parts for mobile communication.
Dielectric compositions known to be usable for stacked type element operable with frequencies of MHz or higher are exemplified by CuO or V.sub.2 O.sub.5 -added Bi.sub.2 O.sub.3 --CaO--Nb.sub.2 O.sub.5 and glass-added (Mg, Ca)TiO.sub.3, (Zr, Sn)TiO.sub.4 or (CaO--ZrO.sub.2).
CuO or V.sub.2 O.sub.5 -added Bi.sub.2 O.sub.3 --CaO--Nb.sub.2 O.sub.5 compositions may be sintered at 900.degree. C., and have a dielectric constant of 40 or higher and a quality factor of 1,800 or higher. In addition, chip type stacked capacitors, which use low melting point electrodes such as Ag and Cu, and dielectric resonators using strip lines can be manufactured (Japanese Laid-Open Patent Application No. Hei. 11-34231).
Japanese Laid-Open Patent Application No. Hei. 9-315859 discloses that CaO--ZrO.sub.2 is added with alkaline metal compounds including boron, lithium and sodium, and thus can be sintered at 1,000.degree. C. or lower. These compositions, however, have drawbacks of being not effectively sintered at a temperature lower than 1,000.degree. C., being poor in dielectric properties at microwave frequencies, and showing large reactivity with electrode materials.
Another well-known dielectric composition is based on a BaO--TiO.sub.2 --Nb.sub.2 O.sub.5 system, whose subtypes comprise BaTiNb.sub.4 O.sub.13, Ba.sub.3 Ti.sub.4 Nb.sub.4 O.sub.21 and Ba.sub.3 Ti.sub.5 Nb.sub.6 O.sub.28 which is reported to sinter at 1,350.degree. C. and to have a dielectric constant of 28.1 and a temperature coefficient of the dielectric constant (.tau..sub..di-elect cons.) of -21 (see. "Dielectric properties of barium titanium niobates", G. L. Roberts et al., Journal of Materials Research, Vol. 12 (1997), pp. 526-530). Additionally, it is known that, when a small amount of Ta is substituted for Nb, the dielectric constant is increased and the temperature coefficient may be controlled. Recently, it was reported that, when Ba.sub.3 Ti.sub.5 Nb.sub.6 O.sub.28 is sintered at 1,300.degree. C. in microwave bands, a sintering density of 4.6, dielectric constant of 41, quality factor (Q.times.f) of 4500, and temperature coefficient of resonant frequency of 8 ppm/.degree. C. are observed (see. "New low loss microwave dielectric ceramics in the BaO--TiO.sub.2 --Nb.sub.2 O.sub.5 /Ta.sub.2 O.sub.5 system", M. T. Sebastian, Journal of Materials Science; Materials in Electronics, Vol. 10 (1999), pp. 475-478). U.S. Pat. No. 5646080 discloses that a dielectric stable at high temperature comprises a sinterable dielectric ceramic powder composition which can be fabricated into multilayer ceramic capacitors with nickel, nickel alloy, palladium or palladium/silver alloy inner electrodes, the thus formed capacitors having a variation of capacitance with temperature of less than .+-.20% over the range -55.degree. C. to 140.degree. C. as compared to the value at 25.degree. C.
Accordingly, these compositions still need improvements of low quality factors and, in particular, no low temperature sintering properties, though the temperature stability is excellent and the dielectric constants are high.